In the field of orthopedics, the cutting of living bone is a prerequisite for many procedures. Such procedures include the reconstruction of damaged tissue structures due to accidents, the grafting of healthy bone into areas damaged by disease, or the correction of congenital facial abnormalities like a receding chin line. Over several centuries, these tasks were performed through the utilization of devices called bone saws.
Traditional bone saws are categorized into several basic categories. Hand powered saws or drills are just that, hand held devices which require the operator to move the device in a fashion similar to that used for carpentry tools. Powered devices, whether electric or pneumatic, are of either the reciprocating or rotary type. The reciprocating devices use a flat, sword like blade where the back and forth motion is provided by a motor instead of the hand. The rotary devices use a rotating motor to spin a drill bit or a blade which has teeth arranged around its circumference similar to a table saw blade. All of these traditional bone saws are used today in medical procedures around the world.
While traditional saws are functional, they have many disadvantages. With either the band or reciprocating saws, for instance, it is not easy to initiate and direct a cut. A cut must start from an edge or, alternatively, a starting hole must be used. To create a starting hole, a drill or similar instrument is operated to bore into the bone. Subsequently, a cutting blade is inserted into the bored hole. The user can then proceed to cut. Alternatively, a rotary type blade may be used. However, when a rotary blade is used, the cut must follow a relatively straight path to prevent the blade from binding in the cut. With all blades the ability to create a curved or compound angle cut is extremely limited by the blade chosen. The relatively thick blades have a wide kerf; so that a significant thickness of the viable bone is lost in the cutting procedure. Physicians would like this width to be as thin as possible in most procedures where reconstruction is necessary.
Above all, the relatively slow linear or tangential speeds of conventional bone saw blades coupled with the teeth necessary for cutting result in high frictional losses, which becomes manifested as heat. Heat will cause necrosis of the tissue if the bone temperatures reach 47° C. for more than a few seconds. When tissue necroses, the bone recedes after the surgery as the necrotic bone is overgrown. During such natural post-surgical tissue developments, the thickness of the cuts in the bone actually increases. The bone rescission process must be complete before healing can begin. To prevent the shortening of the length of the bone, metal plates and screws are used to fix the bone fragments in proper position. All of these factors obviously lead to increased operative time, and more importantly, to dramatically increased healing time, since the bone must knit across a greater span. Some studies have shown the strength of the bone to be effected negatively as well.
When an upper or lower jaw is to be cut in elective surgery, the heating effect of traditional saws requires even more extraordinary intervention to prevent damage. Cutting the jaw between the teeth will cause loss of teeth if the bone is damaged or does not heal quickly. To prevent the tooth loss, the teeth must be spread apart preoperatively; sometimes forcing the patient to wear braces for up to 6 months before the operation can take place. In these cases, the costs and patient discomfort increases dramatically.
To limit the tissue temperature rise in an attempt to reduce necrosis, some traditional surgical saws provide cooling liquid to the surgical site. See, for instance, U.S. Pat. No. 4,008,720 to Brinckmann et al. These devices typically introduce coolant into spaces between segments on the cutting edge or rely on spray methods to flood the cutting site with fluid. Another technique employed by clinicians is to make very light cuts and increase the time between passes of the tool. Coupled with irrigation of the area, bone temperature rise is reduced measurably. Of course, this technique increases operative time and clinician fatigue.
Several researchers have proposed the use of ultrasonic tools for bone separation. The use of ultrasonic surgical instruments for cutting through various tissues is well known. While these devices are superior to the traditional saws in several aspects such as reduced kerf size, reduced noise, and superior ability for making complex geometric cuts, the temperature rise in bone due to frictional heating at the blade/tissue interface is still a significant problem. The problem is exacerbated with the use of ultrasonics due to the rapid motion involved as compared to that of traditional reciprocating saws. Some designers have tried to reduce heating by modifying the cross-section of the cutting blade. U.S. Pat. No. 5,188,102 to Idemoto, U.S. Pat. No. 4,188,952 to Loschilov, and U.S. Pat. No. 5,261,922 to Hood all show designs for cutting which have modified cross sections to reduce frictional heating.
Several ultrasonic devices have provided cooling to the cutting blade with varied degrees of success. U.S. Pat. No. 4,823,790 to Alperovich et al. shows a design for a cryogenically cooled scalpel blade. However, this design may actually damage viable tissue by freezing. In addition, this design does not provide any coolant to surrounding tissue not in direct contact with the blade.
U.S. Pat. Nos. 5,205,817, 5,188,102, and 4,832,683 all to Idemoto show examples of ultrasonic instruments with provisions for fluid cooling. These instruments, however, either do not provide optimal coolant flow where it is needed, mainly at the cutting portion of the blade, or for ones that do provide coolant at the tip, they interrupt the cutting edge with holes for the coolant. An interrupted, uneven cutting edge hinders manipulation and makes it difficult to guide the blade on the bone surface.
One phenomenon associated with ultrasonic tooling which acts to hinder the beneficial effects of irrigating the operative site is ultrasonic atomization. When an ultrasonically vibrating body is brought into contact with fluid, that fluid is broken into small droplets which have a size inversely proportional to the frequency of vibration. In other words, the higher the frequency, the smaller and more mobile the liquid drop. Droplets created by ultrasonic vibrations can be very small in size, with some being less than 1 micron in diameter. This phenomenon is well known to the art. In fact, many devices intended to atomize liquid, such as room humidifiers, medical nebulizers, and industrial spray nozzle are based upon this principle. In the operating theater, however, the presence of nebulized particles is not appreciated, since these particles may contain viral or bacterial agents. Also, some of the fluid will be atomized before reaching the operative site, reducing the cooling efficiency. An effective way to insure the liquid transport is needed.
Two devices were developed to solve these issues. These devices are described in U.S. Pat. Nos. 6,379,371 and 6,443,969. The blades of these patents are extremely effective not only in maxillofacial applications as described above but also in other applications involving bone cutting as well.
One limitation of these blades is that the blades must be advanced into the bone tissue in a plunging or brush stroke manner, with the distal tip of the blade leading. Sideways or lateral advancement of the blade after the initial cut is made has not been shown to be practical. In addition, after a cut is made, the blades can cut only tissue facing the incision. This mode of operation is not advantageous in cases were extremely sensitive tissue, such as brain or spinal cord tissue, is beneath the bone.